Electrical capacitor with thermal fuse



Dec. 21, 1965 F. w. DANIELS 3,225,276

ELECTRICAL CAPACITOR WITH THERMAL FUSE Filed Sept. 14, 1961 2a //////j// 15' 16a. &\\\\\\\\\\ 8a United States Patent 3,225,276 ELECTRICAL CAPACITOR WITH THERMAL FUSE Franklin W. Daniels, Glens Falls, N.Y., assignor to General Electric Company, a corporation of New York Filed Sept. 14, 1961, Ser. No. 138,141 4 Claims. (Cl. 317256) The present invention relates to electrical conductors having improved electrical contact, and more particularly concerns an arrangement for providing improved electrical contact between an aluminum conductor and another conductor of dilferent composition where the conductors contact one another under light pressure.

While not limited thereto, the present invention has particular application to the thermal fuse arrangement disclosed in co-pending application Serial Number 113,- 459, Paul and Grahame, filed May 29, 1961, now abandoned, and assigned to the same assignee as the present invention.

It has been found that when fusible alloy strips are simply laid in contact with aluminum electrode foils of capacitors as disclosed in the aforementioned co-pending application, difficulty may arise due to high contact resistance between the fusible strip and the aluminum electrode foil. Such poor contact may give rise to panting of the conductors and arcing between them, ultimately leading to failure of the capacitor. In the large scale manufacture of roll-type capacitors, and especially in the use of known automatic winding apparatus in such manufacture, it is usually not feasible to securely join the fuse strip to the foil electrode, such as by crimping, riveting, fusion,

or the like.

It is, accordingly, an object of the invention to provide improved electrical contact between electrical conductors, one of which is aluminum.

[t is a particular object of the invention to provide an improved fuse assembly of the aforementioned type in electrical capacitors.

It is another object of the invention to provide improved electrical contact between electrical conductors, one of which is aluminum, where the conductors are held in contact with only slight pressure.

It is still another object of the invention to provide improved electrical contact between conductors as abovedescribed where the conductors are in contact in a nonoxidizing environment.

Other objects and advantages will become apparent from the following description and appended claims.

With the above objects in view, the present invention relates to an electrical contact arrangement comprising dissimilar electrical conductors adapted to contact one another for making an electrical connection therebetween, one of the conductors being aluminum and the other conductor having a layer of aluminum intimately secured to and overlying its contact surface.

The invention will be better understood from the following description taken in conjunction with the accompanying drawing, in which:

FIGURE 1 is a fragmentary perspective view of a capacitor winding having a fuse assembly in which the invention may be embodied;

FIGURE 2 is a view partly broken away showing the capacitor roll enclosed in a casing;

FIGURE 3 is an enlarged detailed view of the fuse component of the FIGURE 1 device incorporating the present invention;

FIGURE 4 is a sectional view of the fuse assembly arrangement of FIGURE 1 taken along the line 44; and

FIGURE 5 is a magnified detail view of a portion of the contact region of the fuse assembly shown in FIG- URE 4.

FIGURE 1 shows in fragmentary form the inner (i.e., starting) end of a capacitor roll 1 having a thermal fuse assembly in which the invention may be embodied. Capacitor roll section 1 comprises a pair of aluminum electrode foils 2 and 3 separated by and interwound with a pair of dielectric strips 4 and 5 such as kraft paper, the foil and dielectric strips being alternately disposed and convolutely wound into a compact roll. Each dielectric strip may be composed of one or more insulating sheets, and, as shown, the dielectric strips normally have a greater width than the electrode foils 2 and 3 so as to prevent short circuiting between the foils of opposite polarity. Electrical connection to the electrode foils is made by means of lead-in tap strap 6 electrically connected to foil 2 as hereinafter more fully described, and another lead-in tap strap 7 (see FIGURE 2) electrically conneoted t0 electrode foil 3.

Electrode foil 2 comprises portions 2a and 2b separated by a gap which is bridged by fusible conducting member 8 having tap strap 6 firmly secured thereto by spot welding or other suitable means. Normally, electrode foil 3 is not similarly provided with a gap, and its tap strap 7 is simply connected thereto in any suitable manner at a desired point along its length. The above and other capacitor fuse arrangements of the described type are more fully disclosed in the aforementioned co-pending application.

FIGURE 2 shows capacitor roll section 1 assembled in a casing 9 usually of a metal such as steel and immersed therein in a dielectric liquid impregnant 10 of conventional composition, such as chlorinated diphenyl or mineral oil. Casing 9 is hermetically which is provided with insulating bushings 12 and 13 and external terminals 14 and 15 connected respectively to tap straps 6 and 7.

In the assembly of the capacitor components in usual manufacturing procedures, the fuse assembly described including fusible foil strip 8 and tap strap 6 secured thereto is merely laid in overlapping contact with electrode foil 2, and it is held in place there simply by the frictional contact and limited pressure afforded by the winding turns when the capacitor roll is formed. It has been found that such contact and pressure is often insufiicient to ensure an electrical contact of sufiiciently low resistance between the fusible strip and the electrode foil. Fluctuating temperatures and pulsating mechanical motion of the paper-foil roll prevailing in the capacitor while it is energized contribute to the low and variable pressures exerted on the fuse assembly.

In order to overcome this difliculty, there is provided in accordance with the invention a coating of aluminum on the contact faces of fusible member 8, and this arrangement has unexpectedly been found to markedly improve the electrical contact between fuse member 8 and electrode foil 2.

FIGURE 3 shows in perspective the assembly of fuse strip 8 with aluminum contact facing layers 16, 16a prosealed by cover 11.

. 3 vided thereon in accordance with the invention. In the embodiment shown, facing layers 16, 16a are thin aluminum foil sheets which are firmly and intimately joined to the contact surface of fusible member 8 by knurling or the like. While tap strap 6 is shown secured to only one side of fusible strip 8, it may be attached to the other surface, if desired, or, as is often done in practice, a pair of tap straps may be secured to fusible strip 8 with the latter sandwiched between them and the projecting portions of the tap straps welded or otherwise joined together.

FIGURE 4 shows in sectional view the arrangement of fuse strip 8 with tap strap 6 on one side and aluminum facings 16, 16a on the opposite side laid against aluminum electrode foil 2 bridging the gap between its portions 2a, 2b and with aluminum facings 16, 16a contacting the latter portions.

Whereas it is inconvenient or quite difficult to securely join fuse member 8 in direct intimate connection with electrode foil 2 when using conventional automatic capacitor winding machines, the joining of fuse strip 8 with its aluminum contact facings 16, 16a prior to automatic insertion of the fuse-tap assembly into the desired position of the roll is readily and conveniently achieved. The fuse-tap strap assembly with contact facings thereon is readily adaptable to automatic insertion between electrode foil 2 and dielectric strip by mechanical means in the manner that the tap strap is automatically inserted in a capacitor roll as disclosed in Hogue et a1. Patent 2,547,- 644, issued April 3, 1951, and assigned to the same as signee as the present application. Thus, a continuous strip of thermally fusible material may be prepared with a number of tap straps suitably secured thereto at spaced intervals and the thus-prepared strip may be wound up into a roll suitable for mounting on a spindle of a capacitor winding machine. Thereafter, in a manner similar to that disclosed in the aforementioned Hogue et al. patent, the continuous fuse strip may be automatically cut into separate tap-fuse assemblies which are fed at intervals into the capacitor roll being wound, the individual tap-fuse assemblies simply being laid in against the electrode foil without any additional steps being taken to provide electrical contact therebetween.

The application of aluminum facings 16, 16a on the contact surface of fuse strip 8 can be accomplished simply by passing the fuse strip with superposed aluminum facings 16, 16a, between pressure rollers having knurled surfaces. Other methods for joining these members may be used if desired, such as welding soldering with the use of a solder material of melting point lower than that of fuse strip 8, or other mechanical or electrical joining methods. The aluminum facing may also be in the form of a met-allized coating applied to the surface of strip 8, such as by chemical or electro-deposition, vacuum evaporation, etc.

Conducting fuse member 8, as disclosed in aforementioned co-pending application, preferably has a melting temperature of about 100-400 C. and may be composed of a variety of compositions having such a melting temperature range. For example, it may be composed of 9% zinc-91% tin; 100% tin; or an alloy of tin, antimony and lead in various proportions. It will also be understood that instead of being in the form of a metal foil strip, fuse member 8 may comprise a non-conductive carrier sheet such as paper or plastic film on which a coating of electrically conductive, thermally fusible metal :is deposited by any suitable means, the thus-metallized coating being then suitably provided with an aluminum contact facing as above-described.

While facing layers 16, 16a are described as being composed of aluminum, it will be understood that the aluminum need not be in pure form and may, without going beyond the scope of the invention, have impurities therein or have other metals alloyed with it While still providing effective electrical connection under conditions of light pressure in accordance with the principles of the invention.

As is Well known, aluminum bodies naturally and inherently form a very thin oxide film on the surface thereof simply by exposure to air or other oxidizing atmospheres. In the usual processing of capacitors, the capacitors are subjected to elevated temperatures in the presence of moisture and such conditions are particularly conducive to the oxidizing action. Consequently, as shown more clearly in the magnified fragmentary view of FIGURE 5, both aluminum electrode foil 2 and aluminum facing sheet 16 will have thin aluminum oxide films formed thereon. As shown in FIGURE 5, there will then be present in the contact region between fusible strip 8 and electrode foil 2 an aluminum oxide layer 2' formed on electrode foil 2 and a similar aluminum oxide layer 16' formed on aluminum contact facing 16 which is intimately secured to fusible strip 8. The presence of such aluminum oxide films appear to play a significant part in the improved results obtained in accordance with the invention, as explained hereinafter.

While the reasons for the improvement in the electrical connection afforded by the invention are not entirely clear, it appears that the frictional contact even under light pressure between the hard brittle aluminum oxide layers 2' and 16' results in rupture and at least partial removal of the respective oxide layers and enables the substrate aluminum layers to come into direct electrical and physical contact. In the case where no aluminum. facing is applied to strip 8, it appears that an oxide layer is formed on the latter material, such as tin oxide or the like, which is relatively ductile and soft and consequently fails to result in sufficient removal or penetration of the brittle aluminum oxide layer of the electrode foil and is itself not easily ruptured, especially under conditions of light contact pressure, to enable good electrical contact to take place between the fusible strip 8 and aluminum electrode foil 2. Since in accordance with the present invention the aluminum facing sheet 16 is directly mechanically joined to the underlying fusible metal substrate by knurling or the like as indicated by undulating line 8a in FIGURE 5, good electrical contact is thereby achieved between the facing 16 and the fusible metal 8.

Tests were conducted in connection with the invention to determine the characteristics of the contact re sistance between aluminum foil simply laid against vars ous alloy foils such as those composed of tin-zinc, '[11'1- lead-antimony, tin-copper, and tin-antimony, as compared to similar contacting pairs having aluminum con tact facings knurled to the alloy foils. These tests, in which oscilloscope tracings characterizing the electrical contact resistance were observed, demonstrated that the pairs having aluminum facing as described always were characterized by good electrical conduction and stable contact resistance, in contrast to the pairs lacking the aluminum contact facings which exhibited very unstable resistance characteristics from currents as low as 100 milliamperes and up, and showed almost instantaneous changes of 20 to with variations as high as being noted.

Still other tests were performed to compare contacts made in accordance with the invention and other contacts made by simply laying the alloy foils against the aluminum foil. In these tests an electric current was passed continuously through the contact structure. The magnitude of the current was sufficiently high to produce accelerated failure, such failure resulting from 10- calized overheating and melting of the alloy foil at the contact area even though the temperature of the struc-' ture as a whole was less than that which would normally cause melting of the fuse alloy. Failure occurring within a relatively short time interval was therefore indicative of high contact resistance, whereas failure after a relatively prolonged interval indicated low contact resistance. Typical data resulting from applying the aforesaid test procedure to a number of samples are as follows:

CONTACT STRUCTURE EMBODYING ALUMINUM FAOINGS OF INVENTION Time in minutes to produce failure for currents of indicated magnitude] 10.00 Amps. 11.95 Amps. 14.30 Amps.

Min. Min. Min. 3, 282 2, 259 27. 9 8, 553 3, 360 39. 2 8, 553 3, 444 48. 9 10, 545 3, 588 52. 6 11, 814 7, 818 101.1 11, 958 147. 4

CONTACT STRUCTURE WITHOUT FACINGS 10.00 Amps. 11.95 Amps. 14.30 Amps.

Min. Mm. Min.

The advantage of the contact structure of the inven tion is readily apparent from the above data.

Optimum results are obtained in accordance with the invention when the electrical contact arrangement i in a substantially non-oxidizing atmosphere or environment. Under such conditions, little or no further oxidation of the contacting metals takes place, which action might tend to increase particularly the thickness of the aluminum oxide layer and thereby increase the cont act resistance of the electrical connection. Use of the described contact arrangement in conventional electrical capacitors wherein the capacitor roll is immersed in dielectric liquid thus provides extremely statisfactory results in such application.

Although the invention has been described mainly in connection with a thermal fuse assembly for electrical capacitors, its application is not limited thereto and it may be advantageously used in numerous and widely varied devices where good electrical contact is sought between an aluminum conductor and a conductor of a different composition. In essence, the invention i principally useful for electrical connections and contacts where the contact pressure is relatively light and variable and where it is not feasible to directly join the dissimila-r conductor to the aluminum member.

While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the scope of the invention. Therefore, the appended claims are intended to cover all such equivalent variations as come within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electrical capacitor comprising, in combination, a convolutely wound roll formed of a pair of insulated electrodes at least one of which is aluminum, and fuse means electrically connected to said aluminum electrode in said roll, said fuse means comprising a fusible conductor having a composition different from the aluminum electrode and being electrically connected to said aluminum electrode through opposed surfaces thereof having contact solely produced by the pressure exerted by the capacitor roll winding, said fusible conductor having a coating of aluminum overlying and intimately joined to its contact surface and separating said opposed surfaces.

2. An electrical capacitor comprising, in combination, a sealed casing, a dielectric liquid in said casing, a capacitor section in said casing impregnated with said dielectric liquid and formed of a convolutely wound roll of a pair of insulated electrodes at least one of which is aluminum, and fuse means electrically connected to said aluminum electrode in said roll, said fuse means comprising a fusible conductor having a composition different from the aluminum electrode and being electrically connected to said aluminum electrode through opposed surfaces thereof having contact solely produced by the pressure exerted by the capacitor roll winding, said fusible conductor having a coating of aluminum overlying and intimately joined to its contact surface and separating said opposed surfaces.

3. An electrical capacitor comprising, in combination, a sealed casing, a dielectric liquid in said casing, a capacitor section in said casing impregnated with said dielectric liquid and formed of a convolutely wound roll of a pair of insulated electrodes at least one of which is aluminum, and fuse means electrically connected to said aluminum electrode in said role, siad fuse means comprising a fusible conductor composed of a sheet of fusible metal having a melting point of about -400 C. and being electrically connected to said aluminum electrode through opposed surfaces thereof having contact solely produced by the pressure exerted by the capacitor roll winding, said fusible conductor having a coating of aluminum overlying and intimately joined to its contact surface and separating said opposed surfaces.

4. An electrical capacitor comprising, in combination, a convolutely wound roll formed of a pair of insulated electrode at least one of which is aluminum, and fuse means electrically connected to said aluminum electrode in said roll, said fuse means comprising a tin-containing metal and being electrically connected to said aluminum electrode through opposed surfaces thereof having contact solely produced by the pressure exerted by the capacitor roll winding, said fuse means having a coating of aluminum overlying and intimately joined to its con tact surface and separating said opposed surfaces.

References Cited by the Examiner UNITED STATES PATENTS 11,973,120 9/ 1934 Steinmayer 200 2,210,750 8/1940 Cook et al 339272 X 2,480,280 8/1949 Bergan 339278 X 2,638,521 5/1953 Thomas 200135 X 2,704,341 3/1955 Stacey et al 317-256 2,868,863 1/1959 Cook 339-278 X 2,983,856 5/1961 Martin et al. 317-258 JOHN F. BURNS, Primary Examiner. 

1. AN ELECTRICAL CAPACITOR COMPRISING, IN COMBINATION, A CONVOLUTELY WOUND ROLL FORMED OF A PAIR OF INSULATED ELECTRODES AT LEAST ONE OF WHICH IS ALUMINUM, AND FREE MEANS ELECTRICALLY CONNECTED TO SAID ALUMINUM ELECTRODE IN SAID ROLL, SAID FUSE MEANS COMPRISING A FUSIBLE CONDUCTOR HAVING A COMPOSITION DIFFERENT FROM THE ALUMINUM ELECTRODE AND BEING ELECTRICALLY CONNECTED TO SAID ALUMINUM ELECTRODE THROUGH OPPOSED SURFACES THEREOF HAVING CONTACT SOLELY PRODUCED BY THE PRESSURE EXERTED BY THE CAPACITOR ROLL WINDING, SAID FUSIBLE CONDUCTOR HAVING A COATING OF ALUMINUM OVERLYING SAID INTIMATELY JOINED TO ITS CONTCT SURFACE AND SEPARATING SAID OPPOSED SURFACES. 